Molded plastic parts are widely used in automobiles, trucks, household appliances, toys and the like. Frequently these plastic parts are made from polyolefins such as polyethylene, ethylene copolymers, polypropylene, propylene copolymers, and polyolefin blends. One such blend is thermoplastic olefin (TPO), which is rubber-modified polypropylene. Frequently, these plastic parts have to be painted in order to match the color of painted metal parts which are also present in the automobiles, appliances, or toys. Generally, paints do not adhere well to these plastic parts due to their wax-like surface and their chemical inertness.
One approach to improving the adherence of paint to polyolefins is to use an adhesion promoting composition as a separate primer coating between the polyolefin substrate and the paint. The primer coating adheres adequately to both the polyolefin and the paint and thereby creates a unitary three component structure with the paint as the outer portion of the structure. Alternatively, an adhesion promoting primer composition may be added directly to the paint as a "stir in" adhesion promoter.
Certain carboxylate modified polyolefins have been found to be useful as adhesion promoting primers. Generally, these modified polyolefins have been formulated into solvent-based and water-based adhesion promoting primer compositions. The surface of the part is "primed" by application of the adhesion promoting primer composition and the solvent or water is allowed to evaporate. The surface so primed is then readily coated with any of a variety of different coating compositions to provide a strongly adherent and durable coating.
Conventionally, adhesion promoting primer compositions have been formulated with polyolefins which have been modified with unsaturated carboxylic acids or acid anhydrides. In some instances, such as in U.S. Pat. Nos. 3,579,485, 4,070,421 and 4,954,573, carboxylic acid modified polyolefins have themselves been chlorinated to further improve their primer properties. Chlorinated polyolefins, however, can be expensive to produce and their toughness and durability depends upon the amount of chlorination provided, which can vary. Additionally, some chlorinated polyolefins exhibit only limited solubility in anything other than aromatic or chlorinated solvents. Additionally, the presence of chlorine in the chlorinated polyolefin may pose environmental concerns as to the recyclability and disposal of the chlorine coated materials.
Another approach that has been taken for coating polyolefins involves primer compositions containing carboxylate modified polyolefins which are solubilized in an organic solvent. For example, U.S. Pat. No. 4,299,754 and Canadian Patent 835,910 disclose the coating of polyolefins with compositions containing carboxylate modified polyolefins which may be solubilized in a solvent, such as an aromatic or aliphatic hydrocarbon or a chlorinated hydrocarbon. Unfortunately, such compositions have not always proven effective for coating molded plastic parts.
Other primer-based compositions have attempted to avoid the use of solvents in forming modified polyolefin primer compositions. Typically such compositions are water-based primer coating compositions which require the addition of an emulsification agent, such as a nonionic or anionic surfactant. While such primers may be suitable for certain applications, water-based primers generally suffer from water-sensitivity problems. For example, water-based compositions typically exhibit poor adhesion when immersed in 40 .degree. C. water or subjected to high temperatures with high humidity.
Thus, there exists a need for a primer composition having a modified polyolefin which exhibits good solubility in non-aromatic solvents and which provides suitable adhesion of paints to polyolefin substrates.